Abstract
The crystal structure of manganpyrosmalite has been determined. There are two chemical units of (Mn, Fe)8Si6O15(OH, Cl)10 in the unit cell of symmetry P 3 m 1 with a=13.42, c=7.159Å. The reciprocal lattice constructed from Weissenberg photographs revealed a marked substructure corresponding to the pyrochroite layer, suggesting that the structure contains the same type of layers. The structure analysis was therefore initiated with this assumption in mind, and a complement structure consisting of Si and O was determined by a partial Patterson synthesis. The structure thus determined revealed that a silicate sheet of a new type is contained, which is composed of 12-, 6- and 4-membered rings of SiO4 tetrahedra with the ratio of 1:2:3, and the whole structure is built up of layers composite of a pyrochroite-like Mn octahedral sheet and a silicate sheet. Since this structure is the first example of Mn sheet silicates which, owing to the large size of Mn ions, inevitably exhibit considerable misfits between octahedral- and tetrahedral sheets, some details of deformations of these sheets are described.
The structures polytypic of this crystal and having three-fold axes are derived and possible polytypic relations between manganpyrosmalite-pyros-malite and schallerite-friedelite are discussed.
